Anaerobic digestion apparatus methods for anaerobic digestion and for minimizing the use of inhibitory polymers in digestion

ABSTRACT

The invention includes an anaerobic solids digestion apparatus comprising a digester, at least one draft tube; at least one nozzle and a biogas source; a method for digesting a waste stream in an anaerobic solids digestion apparatus comprises feeding a waste stream to a digester; reacting the anaerobically biodegradable material in the waste stream with anaerobic bacteria in the digester; introducing a mixed liquor into the digester and mixing the mixed liquor; and a method for minimizing the use of inhibitory polymers by concurrently digesting and concentrating the mixed liquor in the digester.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/294,805, entitled “Shear Enhanced Anaerobic DigestionApparatus,” filed May 31, 2001. The entire disclosure of U.S.Provisional Patent Application No. 60/294,805 as filed is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] The invention relates to anaerobic biological treatment of wastestreams with high solids content. Anaerobic biological treatment hastraditionally been applied to the digestion of primary and secondarysludge at municipal sewage treatment facilities, but is also applicableto municipal solid waste, agricultural manures and crop residues, orindustrial solid wastes and slurries where a significant portion of thesolids material is potentially biodegradable.

[0003] Anaerobic digestion of municipal sludge has been performed fordecades to reduce volume, stabilize highly-putrescible material anddestroy pathogens. Conventional digestion is a once-through processwhere the sludge resides in the digester for 20 to 40 days to achieveoptimal digestion. This is expressed as solids retention time (SRT)which in a once-through system is equal to the hydraulic retention time(HRT). SRT represents the average time that solids reside in thedigester, and HRT represents the average time that liquids reside in thedigester. In order to optimize the digestion process and to reduce thesize of the digester vessel, there is a need for an improved digestionmethod that can operate effectively at a reduced SRT.

[0004] One problem associated with municipal sludge digestion is thelarge volume required for the anaerobic digester. Concentrating thesolids in municipal sludge upstream of the digester has been used forreducing the digester volume. Even though municipal sludge is relativelyhigh in suspended solids compared to many industrial wastewaters,typically approximately 99% of the municipal sludge may be water. Toachieve the conventional SRT, the digester must accommodate the volumeof water in the sludge. By concentrating these sludge solids by a factorof two, the digester volume required for digestion could be halved.Traditionally, a thickening process has been applied upstream of thedigester to increase the percentage of solids in the feed to thedigester. Traditional methods involve mechanical thickeners, dissolvedair flotation or similar equipment to concentrate the solids.

[0005] Additives, such as polymers, have been mixed with the sludgestream to enhance the thickening process. These polymers are known inthe art and include, for example, cationic polyacrylamides in awater-in-oil emulsion, solution mannich polymers—nonionic polyacrylamidepolymers made cationic by reacting the amide groups along thepolyacrylamide backbone with both a dialkylamine and a formaldehydesource, and cationic water-soluble polymers in emulsions, for examplepolyamine or poly (diallyldialkylammonium halides). The applicants havediscovered that such polymers can inhibit anaerobic biologicaldigestion. This effect might not be noticeable in conventional systemswith long SRT and relatively low biological activity. However, as thedigestion process is optimized and the SRT is reduced, this impactbecomes more noticeable and prevents achievement of optimal digestionperformance.

[0006] Less conventional methods for thickening the sludge such asmembrane separation have also been used upstream of the digester.However, the hydrophilic nature of the solids in the waste stream makesit difficult to extract water efficiently using a membrane separator andpromotes fouling of membranes, a build up of colloidal hydrophiliccompounds which is difficult to penetrate and disturb. Traditionally,this made membrane separation an unattractive method for thickening thewaste stream. Thus, in order to optimize anaerobic sludge digestion,there is a need for an improved method of concentrating the feed stockdelivered to the digester and eliminating the need for the above notedpolymers in optimized digesters.

[0007] Digesters for the anaerobic digestion of municipal publicly-ownedtreatment works (POTW) sludge are generally large tanks of relativelylow height providing for 20-40 days of HRT. Proper treatment ofmunicipal POTW sludge requires a sufficient inventory of activedigesting bacteria and contact of those bacteria with the biodegradablefraction of the sludge. Contact is achieved by mixing digester contents.Optimally, the digester contents are mixed thoroughly. Conventionalmixing methods include mixing by mechanical methods and mixing by usinggas. However, the large and low design of conventional digesterstypically results in “dead zones” which are not mixed and which couldreach or exceed approximately 15% of the digester.

[0008] An “egg-shaped” digester has been developed to address theseproblems. This shape has improved the overall performance by effectivelyapproaching a 100% mixed digester volume. This digester also requires asmaller widest cross-sectional area because it is taller relative to thetraditionally-shaped tanks noted above. However, construction ofegg-shaped digesters must overcome complex geometry. Although they aresmaller than conventional digesters, they are still relatively large andexpensive to construct. These structures improve mixing efficiency, butremain limited by the solids retention time (SRT) dictated by theirdesign parameters and the typical biological activity of a continuouslystirred system.

[0009] Accordingly, there also remains a need for improved digesterperformance by exposing more surface area of the degradable organics andavailable digesting bacteria to increase the opportunity for reactionsbetween them. One way to achieve this is to fragment the sludgeparticles so as to expose degradable organics and digesting bacteria onthe interior of the particles. These components may then be brought intocontact in a high-energy environment. This requires turbulent mixing inthe digester.

[0010] One method of mixing in an anaerobic sludge digester is the loopdigester. Loop digesters have a continuous circulating flow which may bearound a draft tube configuration. A mixing method used in the field ofaerobic digestion is the concept of an eductor nozzle immersed in aliquid filled vessel. The pressure on the pumped side of the nozzle canbe used to accelerate the flow of liquid at the nozzle outlet thusreleasing energy into the liquid filled vessel and disturbing the vesselcontents to effect mixing. Additionally, this acceleration creates asuction effect (similar to a Venturi) which can be used to draw asecondary fluid or gas into the flow stream.

[0011] Eductor nozzles to fragment biological solids have been used inthe treatment of wastewaters using high rate aerobic digesters thatapply a shearing force to the mixed liquor in the digester. With thesupplemental addition of oxygen in the form of air injection, thesedigesters rely on contact between wastewater and biomass particles in anoxygen-rich environment to promote aerobic bacterial digestion ofsoluble components contained in the wastewater.

[0012] Another problem associated with municipal sludge is its disposal.Regulatory restrictions on the disposal of sludge make it desirable thatthe sludge be treated to “Class A” standards prior to disposal. 40 CFR§503.32 proscribes EPA standards regarding the use and disposal ofsewage sludge and is incorporated herein by reference. Accordingly thereis a need for an improved digester design that can provide foroperational or process modifications that achieve sludge which istreated to Class A standards.

BRIEF SUMMARY OF THE INVENTION

[0013] The invention includes an anaerobic solids digestion apparatuscomprising a digester; a mixing device in the digester capable ofdirecting a flow of a mixed liquor within the digester; and a shearingdevice in communication with a mixed liquor inlet to the digester, theshearing device being capable of imparting shear to a mixed liquorwithin the digester.

[0014] The invention also includes an anaerobic solids digestionapparatus comprising a digester comprising a shear source capable ofimparting shear to a mixed liquor within the digester and a concentratorin fluid communication with a mixed liquor inlet of the digester and atleast one mixed liquor outlet of the digester, wherein the concentratorand digester are configured to allow for concurrent concentration anddigestion of a mixed liquor.

[0015] The invention also includes an anaerobic solids digestionapparatus comprising a digester; at least one draft tube positioned inthe digester and capable of directing a flow of a mixed liquor andcomprising an upper inlet and a lower outlet; at least one nozzlecomprising a gas inlet, a liquid inlet, an outlet and an interiorsurface, the nozzle further comprising a gas tube having an exteriorsurface, the tube extending from the nozzle gas inlet to the nozzleoutlet, wherein a generally annular space is defined between theexterior surface of the gas tube and the interior surface of the nozzle;and a biogas source in communication with the gas inlet of the nozzle.

[0016] The invention additionally includes a method for digesting awaste stream in an anaerobic solids digestion apparatus, the methodcomprising feeding a waste stream comprising anaerobically biodegradablesolids to a digester; reacting the anaerobically biodegradable solids inthe waste stream with anaerobic bacteria in the digester to reduce anamount of the biodegradable solids, thereby producing a mixed liquor anda biogas; introducing a mixed liquor to the digester through a shearingdevice; and mixing the mixed liquor within the digester.

[0017] The invention includes a method for improving the efficiency ofan anaerobic solids digestion apparatus comprising feeding a wastestream comprising anaerobically biodegradable solids to a digester;reacting the anaerobically biodegradable solids in the waste stream withanaerobic bacteria in the digester to reduce an amount of thebiodegradable solids, thereby producing a mixed liquor and a biogas;imparting a shearing force to the mixed liquor in the digester; andconcentrating the mixed liquor, wherein the steps of reacting theanaerobically biodegradable solids and concentrating the mixed liquoroccur concurrently.

[0018] The invention additionally includes a method for minimizing theneed for use of a polymer that inhibits biological activity in a wastestream in a digestion apparatus, the method comprising feeding a wastestream to a digester, wherein a portion of the waste stream isbiodegradable; reacting the biodegradable portion in the waste streamwith bacteria in the digester to produce a mixed liquor and gas; andconcentrating the mixed liquor with a membrane separator, wherein thesteps of reacting the biodegradable portion in the waste stream andconcentrating the mixed liquor occur concurrently.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0019] The foregoing summary, as well as the following detaileddescription of preferred embodiments of the invention, will be betterunderstood when read in conjunction with the appended drawings. For thepurpose of illustrating the invention, there is shown in the drawingsembodiments which are presently preferred. It should be understood,however, that the invention is not limited to the precise arrangementsand instrumentalities shown.

[0020] In the drawings:

[0021]FIG. 1 is a schematic representation of the features of a shearenhanced anaerobic digestion apparatus according to the invention;

[0022]FIG. 2 is a schematic representation of a two-phase mixing andshearing nozzle.

[0023]FIG. 2a is a schematic representation of a single phase mixing andshearing nozzle.

[0024]FIG. 3 is a schematic representation of a conventional sludgetreatment process;

[0025]FIG. 4 is a schematic representation of a sludge treatment processaccording to an embodiment of the invention which includes a membraneconcentrator;

[0026]FIG. 5 is a schematic representation of an anaerobic digestionapparatus and a membrane concentrator; and

[0027]FIG. 6 is a schematic representation of an alternative anaerobicdigestion apparatus and a concentrator.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention relates to an apparatus, designatedgenerally in the drawings as 100, and process for the anaerobicdigestion of solids in a waste stream using a shear enhanced anaerobicdigestion apparatus (SEAD). The invention also relates to an apparatusand method for concurrently concentrating and digesting the degradablesolids fraction of a waste stream. The invention additionally relates toa method of minimizing the need for use of a polymer(s) that can inhibitbiological activity in a waste stream. By utilizing a preferredcontinuous recirculating flow around a draft tube, mixing of the systemcan be achieved without moving parts within the digester. A shearingnozzle may be used in the apparatus to impart energy to the digestercontents so as to fracture solids particles and expose the maximumreactable surface area. When energy is released through the nozzle, thefluid inside the draft tube is accelerated, resulting in about aten-fold increase in internal flow rates compared to the recirculationflow, as described further herein. Shearing of the particles in thewaste stream or waste slurry solids, occurs both within the nozzle, aswell as in a turbulent mixing zone at the outlet of the nozzle,physically breaking down the solid biodegradable particles of the wastestream or waste slurry into smaller particles, exposing more surfacearea of the biodegradable solids. Biogas produced in the anaerobicdigestion can be recycled in the two-phase nozzle and mixed with themixed liquor. Entrained biogas is dispersed throughout the circulatingmixed liquor providing for maximum turbulence in the mixed liquor toensure optimal contact of the biodegradable organics with the anaerobicbacteria. These factors improve mass transfer rates resulting insignificantly improved reaction efficiency evidenced by reduced SRT.

[0029] A fraction of the digester contents can be concentrated by usingmembrane separation or other conventional concentration process incombination, and preferably concurrently, with the digestion apparatus.A concentrator is in fluid communication with an inlet and an outlet ofthe digester, so that the concentrator and digester may operateconcurrently. As digestion acts on the degradable waste material in thedigester, the hydrophilic components are being digested, and thedigester contents are more easily concentrated. Because the apparatuscan work alongside a concentrator, they can be operated concurrently andthe thickening of the waste is significantly improved. Thus, the designof the apparatus including a concentrator as described herein enhancesseparation performance over conventional concentration processes withthe result that the feed stock to the digester is effectivelyconcentrated. Additionally, the performance of the digester is enhancedbecause the concentration can be performed without the need for the useof polymers that can inhibit biological activity or other additives thatcan negatively impact an optimized digester such as the digesterdescribed herein. Applicants have additionally discovered thatconcurrently concentrating while digesting the feed stock for othertypes of digesters, including, for example, non-shear enhanced anaerobicdigesters, egg-shaped anaerobic digesters, and aerobic digesters, canalso minimize the need for additives, such as thickening polymers thatmay act to inhibit biological activity in those digesters whenoptimized.

[0030] In general, waste streams to be treated according to the variousembodiments and aspects of the present invention may be any wastestreams containing material that is at least partially biodegradable byanaerobic or aerobic bacteria. However, the principle aspects of theinvention are directed to anaerobic bacteria unless otherwise specifiedherein. Preferably, such waste streams are primary and/orwaste-activated sludge from municipal sewage treatment plants orindustrial aerobic wastewater treatment plants, but may also includewastewater streams from solid waste agricultural manures, crop residues,industrial solid wastes, sludge, and slurries or any other high solidswaste streams where a significant portion of the solids material ispotentially biodegradable. The present invention can process streamswith high concentrations of total suspended solids (TSS) and/or fat, oiland grease (FOG), as well as the slurries or solid waste havinganaerobically digestible material.

[0031] In a preferred embodiment of the invention represented in theattached Figures, the waste stream is preferably a waste slurry. Morepreferably, the waste slurry is a mixture of primary and secondarysludge in a ratio of about 70:30 to about 30:70, most preferably about60:40 to 40:60. In the case of waste sludge, a total solidsconcentration of about 2 to about 20 wt % is preferred for embodimentsof the digester of the invention without the concentrator based on theweight of solids divided by the weight of the sample. For embodiments ofthe digester with the concentrator, the total solids concentration ispreferably about 1 to about 10 wt %, more preferably about 1 to about5%. About 60 to about 90 wt % of the solids present in such waste sludgeare generally volatile, potentially biodegradable solids. The wastesludge will also preferably have an inlet chemical oxygen demand (COD)level of at least 20,000 mg/l for the embodiments of the digester of theinvention without the concentrator and at least 5,000 mg/l, morepreferably at least 10,000 mg/l, for embodiments of the invention withconcentrator. For waste slurries, COD levels of greater than 2,000 mg/lare preferred. It will be understood, however, based on the disclosurethat the anaerobically digestible stream processed by the invention canhave varied characteristics.

[0032] The following is a detailed description of preferred embodimentsof the invention and should not be considered to be limiting. Thereferenced schematics in FIGS. 1-6 are representative and not drawn toscale. Certain terminology is used in the following description forconvenience only and is not considered to be limiting. The words “lower”and “upper”, “top” and “bottom”, “upward” and “downward” and “left” and“right” designate directions in the drawings to which reference is made.The terminology includes the words specifically mentioned, derivativesthereof and words of similar import.

[0033] A waste stream can be introduced into a digester 30, as shown inFIG. 1, through one or more inlets 2 located around the periphery of thedigester vessel or through the digester recirculation conduit 60, 64 asdescribed herein. The digester 30 also preferably includes one or moremixed liquor outlets 50 around the periphery of the digester vessel, oneof which may serve as a main outlet for discharging digester effluent orthe digested liquor or sludge for disposal or further processing. Thedigester may also include one or more sample ports 44 around theperiphery of the digester. The digester 30 additionally may include oneor more drains 40 for emptying the vessel when not in use. The term“mixed liquor” in this specification includes, but is not limited to amixed liquor, a mixture of solids, liquids and gas and the biodegradableportion therein and bacteria therein, which in the preferred embodimentare anaerobic bacteria. The mixed liquor may be within the digester, fedinto the digester as a recycle stream from the digester and any effluentfrom the digester.

[0034] Preferably, the digester volume is selected such that the sludgeretention time SRT within the digester is about 2 to about 20 days,preferably about 6 to about 12 days. The configuration of the digestermay vary, however, preferably it is a generally cylindrical vessel witha height as measured along the longitudinal axis A of the digester ofpreferably about 25 feet (8 m) to about 75 feet (23 m), and morepreferably about 45 feet (14 m) to about 55 feet (17 m), with a heightto diameter ratio of about 0.2 to about 20, and more preferably about 1to about 4, wherein the diameter is measured in a transverse directionalong the largest transverse dimension of the digester. The digester canbe constructed of any conventional material that is consistent with thematerials handling and structural requirements of the particularmaterials to be digested and digester design chosen. However, it ispreferable that the digester is constructed of concrete, steel orfiberglass.

[0035] As shown in FIG. 1, the waste stream is preferably fed to thedigester upstream of a recirculation pump 6. The digester inlet 2 ispreferably upstream of the digester recirculation pump 6 so that thefeed immediately and intensely contacts with the digesting bacteriaexisting in the preferred recirculated stream flowing through conduit 60and is mixed proportionately with that stream. In the preferredembodiment, the waste stream is fed continuously to the digester 30,although a batch feeding operation may also be utilized. The rate offeed of waste stream into the digester may vary, but the maximum feedrate can generally be determined by dividing the volume of the digesteremployed by the design hydraulic retention time (HRT).

[0036] Preferably the digester is operated at a controlled mixed liquorvolume, which is a substantially constant volume, subject to typicalcontrol fluctuations. To maintain the mixed liquor volume inside thedigester at a controlled volume, an amount of mixed liquor substantiallyequal to the flow rate of the waste stream feed is extracted from thedigester 30 via a mixed liquor outlet 50, preferably one located toextend generally transversely from a side 32 of the digester vessel. Acontrol valve 4, such as a gravity overflow or control valve, or anyother appropriate flow control mechanism can be used to control themixed liquor discharge so as to maintain the liquid volume in thedigester at a substantially constant volume.

[0037] In the preferred embodiments illustrated in FIGS. 1 and 5, theapparatus further includes a recirculation system 46, including therecirculation pump 6 and recirculation conduit 60, 64. The recirculationsystem provides fluid communication for the mixed liquor L within thedigester 30 from a mixed liquor outlet 50, which in this instance servesas a mixed liquor recirculation outlet, preferably in the bottom 52 ofthe digester vessel, to a liquid inlet 65 of at least one preferredtwo-phase nozzle 18 as best shown in FIG. 2. In the preferredembodiment, the rate of recirculation through the recirculation systemis controlled to be substantially constant such that the digester volumedivided by the digester volume recirculation rate is about 15 to about150 minutes, and more preferably about 45 to about 75 minutes. Similardigester recirculation rates would be preferred in the processing ofother waste slurries such as agricultural or industrial slurries.However, recirculation rates can be altered or optimized for varyingsystems. A first conduit 60 conveys liquid from pump 6, which may be apump or any liquid pumping apparatus that moves the liquid through theconduit 60, 64, to the liquid inlet 65 of the nozzle 18. A secondconduit 64 conveys liquid from the mixed liquor outlet 50 that serves asthe mixed liquor recirculation outlet to the pump 6. It is understoodthat the term “conduit”, as described in this specification, may be anypipe, conduit, tube, conveyance mechanism, valve, or indirect or directconnection or the like which provides fluid communication as describedherein.

[0038] If the inlet 2 is connected to the second conduit 64, therecirculation system 46 provides a continuous blend of waste stream feedand recirculated mixed liquor from the digester to the liquid inlet 65of the preferred two-phase nozzle 18 which would then discharge freshwaste stream and recirculated mixed liquor and biogas from a biogassource into the digester 30. The nozzle 18, best shown in FIG. 2,includes a nozzle gas inlet 13 in fluid communication with a biogassource, a nozzle liquid inlet 65 and a nozzle outlet 20. The nozzlefurther has a gas flow tube 15 extending from the nozzle gas inlet 13 tothe gas tube outlet 33 in proximity to the nozzle outlet 20. A nozzlespace 56 which is generally annular is defined by the exterior surface35 of the gas flow tube 15 and the interior surface 54 of the nozzle 18,through which recirculated mixed liquor is passed.

[0039] As illustrated in FIG. 1, the pump 6 circulates the recirculationstream and/or feed stream and pressurizes the slurry upstream of thepreferred two-phase nozzle 18. The pump energy is transferred to thedigester contents at the fluid outlet 20 of the preferred two-phasenozzle 18. The fluid outlet 20 of the preferred two-phase nozzle 18 isconfigured such that the nozzle annular space 56 narrows at the nozzleoutlet causing an acceleration of the mixed liquor at the outlet. Thevelocity gradient generated via the nozzle outlet 20, is preferablymaintained at a level of about 50 to about 500 sec⁻¹, as defined byformula (I)

G=(P/(μV))^(0.5)  (I)

[0040] wherein G is the mean velocity gradient, sec⁻¹, P is the powerrequirement in Watts, V is the digester volume in cubic meters, and μ isthe dynamic viscosity of the digester contents in Ns/m².

[0041] The energy transferred to the mixed liquor at nozzle outlet 20imparts a shearing force on the solids in the stream which breaks thesolids into smaller particles and increases the surface area. Theincrease in surface area exposes more unreacted organics making themaccessible to the anaerobic bacteria. The shearing occurs both insidethe nozzle 18 as the fluid is accelerated and in the mixing zone 66outside the nozzle outlet 20 where the energy of the mixed liquor istransferred to the digester contents.

[0042] In the preferred embodiment, the nozzle is a two-phase nozzlewhich can provide a shearing force as well as educting a gas. However,it will be recognized by one skilled in the art that a single phasenozzle 72, as shown in FIG. 2a, would also be suitable for the providingshear to the mixed liquor in the digester. The nozzle in FIG. 2aincludes a liquid inlet 73 having a diameter d₂, which would be in fluidcommunication with a mixed liquor outlet 50 of the digester 30 shown inFIG. 1, and a liquid outlet 74 having a diameter d₃ which is narrowerthan the diameter d₂ of liquid inlet 73 for introducing mixed liquorinto the digester, wherein the diameters d₂ and d₃ are measured in thelargest dimension and transversely across openings 73 and 74,respectively. The narrower outlet 74 causes acceleration of the mixedliquor at the outlet 74. Recirculated liquor passes through an interiorspace 75 of the nozzle 72 defined by interior walls 76 of the nozzle 72.Unlike the two phase nozzle 18 described above, the single-phase nozzle72 does not include a gas tube. Additionally, other types of shearingdevices, for example a Venturi valve or nozzle, or an impeller, capableof fracturing the solids and introducing them into the digester whilenot as preferred as the nozzles of the present invention may also beused within the scope of the invention.

[0043] The number of nozzles used can vary depending upon the volume ofthe digester and/or the desired optimized process. For example, twonozzles are shown in the preferred embodiment of FIG. 5. Preferably,about one nozzle to about 150 to about 1,500 cubic meters of digestervolume is preferred, and more preferably about one nozzle to about 600to about 900 cubic meters of digester volume.

[0044] A “mixing device” which is preferably a draft tube but may be anymixing device including impellers, injected gas, vacuum pumping, mixingblades and the like capable of inducing mixing within the digester arewithin the scope of the invention. It will be recognized by one skilledin the art that other forms of mixing such as mechanical mixers or gasinjection or other mixing methods in the presence of a shear enhancedmedium would contribute to an improvement in mass transfer rates and,accordingly, be suitable for use in the invention. As best shown in FIG.1, the nozzle outlet 20 discharges proximate to the inlet 22 of a drafttube 28 positioned below the level 48 of the mixed liquor in thedigester. The fluid exiting the nozzle flows downwardly through theinner area 26 defined by the draft tube 28. The diameter d₁ of the drafttube 28 as measured transversely through the tube is preferably constantand is preferably about 40 to 200 centimeters. Preferably the length l₁of the draft tube 28 as measured along a longitudinal axis B of thedraft tube should be such that the draft tube inlet 22 is sufficientlyfar below the surface of the liquid level to allow circulation of thecontents of the digester into the draft tube inlet 22, as shown in FIG.1, and the draft tube outlet 43 is sufficiently far above the bottom ofthe digester to minimize flow restriction or excessive pressure drop.More preferably the length 11 of the draft tube 28 ranges from about 50%to about 90% of the digester liquid depth Q as measured longitudinallyfrom the bottom 52 of the digester to the surface 48 of the mixedliquor, with the depth of the liquid above the draft tube inlet being nogreater than the liquid depth below the draft tube outlet 43. It isunderstood that more than one draft tube may be employed according tothe considerations discussed above with respect to the number of nozzlesemployed. A 1:1 relationship between the number of nozzles employed andthe number of draft tubes employed is preferred. However, there may bemore than one nozzle per draft tube in the anaerobic digestion apparatusaccording to the invention.

[0045] The continuing downward flow of the nozzle effluent into thedraft tube 28 induces a generally downward flow inside the draft tube.As the mixed liquor exits the draft tube outlet 43, it is forcedupwardly by the digester bottom such that a circulation pattern isdeveloped within the digester 30 in which liquid flows back up aroundthe exterior surface 24 of the draft tube 28 and then is pulled and/orpushed downwardly again into the draft tube 28 through the upper inlet22. This induced circulation pattern around the draft tube preferablyexceeds the volumetric flow rate discharged from the nozzle and isbeneficial to the mixing of the mixed liquor, and more preferably theenhanced circulation is about 5 to about 25 times the dischargevolumetric flow rate of the nozzle. The enhanced mixing provided by thepreferred circulation around the draft tube contributes to an increasedmass transfer rate.

[0046] The degree of anaerobic digestion of a particular biodegradablesolid substrate is limited by the organic makeup of that substrate.However the rate at which this digestion can be achieved is affected bythe mass transfer rate. By improving the mass transfer rate, a reductionin the time for achieving digestion can be effected. The inducedcirculation of the mixed liquor within the digester 30 provides enhancedmixing of the digester contents thoroughly dispersing the feed materialand exposing the unreacted organics to the digesting bacteria. Becausethe shearing effect of the nozzle 18 has increased the exposed surfacearea of the unreacted organics, the mass transfer rates of the anaerobicdigestion process are improved over conventional anaerobic digesters.Under the influence of the energy imparted to the digester by thedischarge from nozzle 18, these conditions increase the mass transferrate.

[0047] The anaerobically biodegradable material contained in the wastestream is digested through reactions in the digester 30, where anaerobicbacteria convert the biodegradable material to a biogas whichsubstantially is made up of methane and carbon dioxide, with lesseramounts of other gases, such as hydrogen sulfide. These gaseouscomponents and other similar anaerobic gas byproducts are generallyreferred to herein as “biogas”. The biogas may also contain smallamounts of water vapor, nitrogen and traces of other volatile compoundswhich may be present in the feed or formed during biodegradation. Thecomposition of the biogas by volume percent will vary depending on theparticular digestible organics being processed. Preferred methane levelsin biogas formed in the digester of the invention are in the range ofabout 50 to about 90 volume percent. Preferred carbon dioxide levels arein the range of about 5 to about 45 volume percent and hydrogen sulfidelevels can range from about 200 parts per million (volume) to about 6volume percent. Action of the anaerobic bacteria on the digestibleorganics also results in multiplication of the anaerobic bacteria.

[0048] This apparatus preferably has a biogas source in fluidcommunication with the nozzle 18 to provide biogas to the nozzle 18. Apreferred biogas source is a biogas recycle system generally designated27 which uses a portion of the gas generated in digestion as gas feed tothe nozzle. However biogas or other anaerobic digestion feed gas can beintroduced independently through inlet 37 and/or used together with abiogas recycle system as shown in FIG. 1. Below the upper surface 34 ofthe digester, the level 48 of the mixed liquor is such that there is anarea 3 above the liquor to allow for collection of the biogas thatde-entrains from the mixed liquor at its upper surface 48. The volume ofthe area 3 above the mixed liquor may vary, but there is preferably adistance of about 4 feet (1 m) to about 7 feet (2.5 m), more preferablyabout 5 feet (1.5 m) to about 6 feet (2 m), of space between the uppersurface 48 of the mixed liquor and the upper surface 34 of the digester30 to prevent any foam that is generated from impeding circulation orgas collection. This biogas collection area 3 is preferably in fluidcommunication with nozzle 18 in a biogas recycle system. Preferably, thebiogas recycle system is provided for recycling a portion of the biogasfrom the collection area 3 to the nozzle 18. More preferably this biogasrecycle system 27 comprises a conduit 5. The conduit provides an outletat one end 7 for biogas in the collection area 3. The other end 11 ofthe conduit is in communication with the gas inlet to the nozzle. Theconduit preferably has a control valve 4 for adjusting the rate of flowof the biogas in the conduit 5. Biogas is also preferably vented fromthe collection area, for example via a defoaming hood 17 through biogasoutlet 29, with such venting preferably being controlled, for example bya further valve 67, to maintain a gas pressure in the area 3 of aboutatmospheric pressure to about 50 inches water at 35° C. (12,400 Pa).More preferably, the pressure range will be about 10 inches water at 35°C. (2,500 Pa) to about 20 inches water at 35° C. (5,000 Pa). Any biogasnot recycled through the biogas recycle system 27 may be discharged andmay subsequently be burned as fuel or utilized for other purposes.

[0049] In addition to biogas, additional gases may be introduced to thenozzle 18 through inlet 37. For example, nitrogen feed gas may be routedto the nozzle 18, either for control of strippable toxins, or foraltering the carbon dioxide equilibrium between the biogas and the mixedliquor, thereby affecting the pH of the mixed liquor in the digester.Alternatively, small amounts of air or oxygen may be routed to thenozzle 18 through inlet 37 or a separate gas inlet (not shown) tomodulate the oxidation-reduction potential (ORP) of the mixed liquor.This is desirable since the tendency of undesirable anaerobic bacterialreactions to produce hydrogen sulfide is favored by particular ranges ofoxidation-reduction potential. Hydrogen sulfide is malodorous, corrosiveto certain materials, and toxic to humans and the digesting bacteria. Byadjusting the oxidation-reduction potential of the mixed liquor to aregion outside those favoring hydrogen sulfide production, the level ofhydrogen sulfide present in the mixed liquor may be reduced, therebymitigating one of the less desirable features associated with anaerobicdigestion. This may be facilitated by an ORP meter or gauge 14 coupledto the SEAD system 100, preferably somewhere along the recirculationsystem 46, most preferably along the conduit 60. This ORP meter or gauge14 may signal a motorized control valve such as valve 16 to adjust theflow of gas from inlet 37.

[0050] As previously described, the mixed liquor is accelerated as itexits the nozzle 18, which is in close proximity to the outlet 33 of thegas flow tube 15. This creates an eduction effect useful for thepreferred biogas recycle system which draws the biogas and removes aportion of the biogas from the biogas collection area 3, through nozzle18, and introduces the portion of biogas into the digester 30. As themixed liquor and the biogas exit the nozzle 18, further mixing occursbetween the portion of biogas and the recirculating mixed liquor at thenozzle outlet 20 and the outlet 33 of the gas flow tube with the gascreating increased turbulence at the nozzle discharge. This turbulenceexerts an additional shearing force on the solid particles in the mixedliquor, further fracturing particles and thereby providing additionalsurface area of degradable organics. This additional shearing mechanismfurther enhances the performance of the invention by providing forincreased mass transfer rates as described above when more of thedegradable organics are exposed.

[0051] Increase of the mass transfer rate in the anaerobic processrequires an increase in the exposed surface area available to thedigesting bacteria as well as thorough mixing to assure that masstransfer reactions can occur at an optimum rate. The eduction of theportion of biogas removed from the gas collection area 3 into therecirculating mixed liquor stream at the nozzle 18 entrains fine gasbubbles in the mixed liquor circulating inside the digester 30. Becausethe gas velocity differs from the fluid velocity of the mixed liquor inboth the draft tube 28 interior 26, where the mixed liquor is flowingdownward carrying the entrained gas by overcoming its buoyancy, and inthe area outside the draft tube 28, where the velocities of the mixedliquor and gas are both upward but different, the entrained gas promotesa high degree of turbulence on the sheared particles in the mixedliquor. In conjunction with the induced circulation imparted by thenozzle 18 and draft tube 28, this entrained gas turbulence furtherpromotes an increase in the mass transfer rate that is beneficial to theoptimum performance of the invention.

[0052] As discussed above, the eduction effect of the nozzle drawsbiogas into the mixed liquor from a biogas source, preferably the biogasrecycle system 27. It is preferred to control the amount of biogasrecycled into the mixed liquor, for example by control valve 4 onconduit 5. In the preferred embodiment the volume ratio of biogas toliquid in the nozzle will be up to about 0.5 of the volume of biogas pervolume of liquid that flows through the nozzle. However, it isrecognized that the characteristics of each mixed liquor will vary formany reasons including the characteristics of the feed waste stream andit is further recognized that these characteristics will impact the rateat which entrained gas generated within or injected into the mixedliquor is released. It is also recognized that as the mass transfer rateof the digester 30 is increased, the rate at which biogas is generatedwithin the mixed liquor mammoth stream due to the digestion process alsoincreases. At such point in the operation of the system where the volumeof biogas entrained in the mixed liquor due to digestion has reached thelevel sufficient to provide the amount of gas turbulence preferred forthe given application, the biogas recycle may be shut off by closingcontrol valve 4.

[0053] The concentration of solids in the waste stream feed is expressedin percent total suspended solids (TSS). In the illustration case ofmunicipal POTW waste sludge, the TSS of the feed before thickening istypically less than 1.0% TSS. Typically this sludge is thickened toabout 5% TSS by utilizing polymers in the thickening process. Theapplicants have discovered that performance of an optimized digester,such as the apparatus of the invention, can be negatively impacted bythe presence of such thickening polymers in the waste stream. Digestingthe unthickened waste stream would effect this, but the digester volumewould become proportionally larger which is not desirable. Thus a methodto thicken the waste feed without the need for polymer addition isdesired.

[0054] In conventional digestion processes, of which FIG. 3 isrepresentative, concentration or thickening of the feed into thedigester occurs upstream of the digester. However, conventional methodstypically do not digest the hydrophilic compounds in the waste stream.The hydrophilic compounds typically present in the solids in the wastestream make it difficult to thicken. The performance of anyconcentration system, and in particular a membrane concentrator, can beimproved if the hydrophilic compounds can be removed from the medium asthese compounds reduce the tendency of the medium to release water. Thedigester apparatus of the present invention will digest thesehydrophilic compounds and, when operated concurrently with theconcentrator, enhances the performance of the concentrator by removingthese hydrophilic compounds that make it difficult for the concentratorto thicken.

[0055] As shown in FIGS. 4 and 5, in a preferred embodiment of theinvention, a concentrator 62 conveys a portion of the mixture of solidsand liquid from a mixed liquor outlet 50 of the digester, preferably themixed liquor recirculation outlet, to the concentrator and back to aninlet 77 of the digester. The concentrator 62 is in fluid communicationwith an inlet 77 and with a mixed liquor outlet 50 of the digester. Morepreferably, the concentrator comprises a pump 70, which may be the pump6 of the recirculation system, but is preferably one or more separatepumps, and a separator 58 having an outlet 53 which is in fluidcommunication with an inlet 77 of the digester and an inlet 68 incommunication with a mixed liquor outlet 50 of the digester. Theconcentrator also preferably includes a conduit 55 with a first end 57connected to the outlet 53 of the separator 58, and a second end 59connected to the inlet 77 of the digester. This conduit preferablyconveys the concentrate from a concentrate side 69 of the separator 58back to the digester 30.

[0056] Most preferably, the separator is a water-permeable membraneincluding and preferably manufactured of a material suitable forprocessing a liquid with various concentrations of suspended solids andsuspended solid particles of varying sizes. An example of a suitablemembrane is an ultra-porous, asymmetric, polymeric ultra-filtrationmembrane. Commonly used polymers include cellulose acetates, polyamides,polysulfones, poly (vinylchloride-co-acrylonitrile)s, and poly(vinylidene fluoride). Membrane separation is an effective concentratingmethod, however, other concentration methods such as a lamellaseparators, dissolved air flotation, gravity belt filter, decanter,rotating screen, or others are suitable separators. The permeate fromthe concentrator and any unused concentrate may be discharged from thesystem or routed to one or more heat exchangers 8 as discussed below.

[0057] If desired, a flow control meter(s) or gauge(s) 51 and preferablya motorized valve 16 may be provided to control the flow of mixed liquorto and from the concentrator. However, any control mechanism isacceptable for controlling the flow of liquor to and from theconcentrator. Additionally, a pressure meter or gauge 63 coupled with amotorized valve 16 may be employed to control the pressure of mixedliquor conveyed to the digester 30 from the separator 58. However, anypressure control mechanism is suitable for controlling the pressure ofthe mixed liquor conveyed to the digester.

[0058] The digester configuration described previously allows forimproved rates of digestion due to increased mass transfer rates but thevolatile solids destruction is limited by the fraction of biodegradablesolids available in said waste stream and is a function of solidsretention time (SRT). The process performance in the same apparatus canbe further improved if the SRT of the digester can be extended withoutincreasing the digester volume. This can be achieved by furtherincreasing the solids concentration in the digester. With the apparatusof the preferred embodiment, concurrent concentration and digestionallows for adjustment of the mixed liquor concentration resulting inincreased SRT in the digester at a fixed waste stream feed rate. Thusthe design SRT can be targeted to achieve a particular goal such as, forexample, to increase volatile solids destruction or to achieve specificeffluent solids concentration. Applicants have discovered thatconcurrent concentration and digestion in the above manner using amembrane separator not only minimizes or eliminates the need to use thepotentially inhibitory polymers in optimized digesters, including thevarious embodiments of the apparatus of the invention, but in anydigester, including non-shear enhanced anaerobic digesters, egg-shapedanaerobic digesters, and aerobic digesters.

[0059] The preferred embodiment is illustrated for the case of amunicipal POTW waste sludge or a waste slurry. Similar digester designsand use of this method are envisioned for digesting other waste slurriesfrom agricultural and industrial sources by the present invention. It isrecognized that the percent fraction of biodegradable material in theslurry will vary based on the source and also that the concentration ofsolids in the slurry could be in the range of about 0.5% to about 12%TSS, but is preferably above 1% TSS. It is further recognized that thetreatment objectives or economics of a given application might make itpreferable to operate the digester 30 or concentrator 62 at parametersoutside of the preferred ranges. Considering these factors it isrecognized that in some applications concentration of the feed may notbe required and, in fact, a dilution stream might instead be preferredto achieve the desired mixed liquor TSS in the digester. It is alsorecognized that this anaerobic digestion process will also digest thesoluble biodegradable organics present in the fluid stream. These andother variations in the present invention are contemplated.

[0060] An optional gas de-entrainment zone 45 can be provided within thedigester 30, as shown in FIG. 5. The gas de-entrainment zone ispreferably in the form of a vertical cylinder defined by a wall(s) 47with an open top and a closed bottom 61, preferably contiguous with thebottom 52 of the digester, except for an opening in the bottom 52 whichis in communication with a mixed liquor outlet 50 of the digester,preferably the mixed liquor recycle outlet. The shape of the wall(s) 47of the zone may be generally cylindrical or otherwise configured so thatthe transverse cross sectional area of the zone is sufficient such thatthe downward velocity of the mixed liquor in the zone caused by thesuction of the recirculation pump 6 is less than the rate of rise of gasbubbles of less than about 1 mm in diameter, in order to allow for suchbubbles to de-entrain from the mixed liquor. The preferred crosssectional area varies, but should be sufficient such that the downwardvelocity of the mixed liquor in the entrainment zone may range from 0.02to 0.2 m/s, more preferably 0.05 to 0.1 m/s. This gas de-entrainmentzone is preferred to avoid any potential for such gas bubbles tocontribute to possible cavitation at the recirculation pump or theconcentrate pump 6, which could result in mechanical damage.

[0061] Alternatively, an optional gas deflector plate 25, depicted inFIGS. 1 and 5 is preferably positioned between the lower outlet 43 ofthe draft tube 28 and the digester bottom 52 to minimize the entrainmentof gas bubbles in the mixed liquor at the point where it enters theconduit 50 of the recirculation system 46 or the concentrator 62.Generally there is no limitation on the shape or materials ofconstruction of the plate. Preferably the gas deflector plate 25 has ashape that is larger than the outlet of the draft tube but smaller thana size that would cause the downward velocity of the mixed liquorflowing around the plate 25 to be increased above the rise rate of gasbubbles less than about 1 mm in diameter, in order to allow for suchbubbles to de-entrain from the mixed liquor. It is understood howeverthat, optional gas de-entrainment zone and deflector plate may used eachalone or in combination in varying SEAD systems according to theinvention.

[0062] Performance of the system can be further enhanced by operatingthe system at optimal levels of pH. Any conventional manual or automatedpH control mechanism can be used to control and optimize theseconditions inside the digester. In such cases a conventional pH controlsystem 10 can be included, preferably in the recirculation conduit 60,to measure pH and dose appropriate amounts of adjusting chemicals. Thepreferred pH level of the digester for the anaerobic digestion is about6 to about 8. For certain waste slurries, such as those with a chemicaloxygen demand (COD) below 30,000 mg/l, adjustment of the pH may berequired to maintain the optimum level in the digester.

[0063] Performance of the system can also be further enhanced byoperating the system at optimal levels of temperature. Any conventionaltemperature control mechanism may be used to control the temperature ofthe mixed liquor in the digester. One mechanism, shown in FIG. 1,includes a temperature meter or gauge 12 and a heat exchanger 8,preferably in the recirculation system 46 of the invention. A preferredmethod, shown in FIG. 5, includes the use of a heat exchanger 8 and asteam injector 9 upstream of recirculation pump 6. As shown in FIG. 5,the heat exchanger 8 may serve as a recovery heat exchanger to captureheat from the permeate from concentrator 62. The waste heat may also berecovered from the digester effluent or the excess concentrate using aseparate heat exchanger (not shown). The temperature control mechanismpreferably heats the feed into the digester (or into the recirculationconduit) to a temperature at or slightly above the preferred reactiontemperature prior to the entry of the mixed liquor and/or feed into aninlet 2 of the digester. The preferred temperature level of the digesterfor mesophilic anaerobic digestion is about 80° F. (25° C.) to about105° F. (40° C.). The digester may also be operated in the thermophilicrange of about 125° F. (50° C.) to about 145° F. (60° C.). However, thefeed into the digester may be heated to any temperature that does notdamage the anaerobic bacteria to a degree that negatively impacts thedigestion process. In the case where it is preferred to destroypathogens in the slurry, such as for the purpose of producing Class Amunicipal sludge, operation in the thermophilic range would allow forachievement of this objective simultaneously with digestion of thedegradable organics.

[0064] As described herein, the recirculation of mixed liquor throughthe nozzle 18 induces a circulation pattern around the draft tube 28,which provides for mixing of the digester contents. In addition, thebiogas entrained in the mixed liquor enhances the mixing. In addition tothe beneficial effects mixing and turbulence heretofore mentioned,mixing provides a more uniform pH and temperature profile across thedigester, thereby maintaining stable reaction conditions within thedigester vessel.

[0065] It has been found advantageous for the desired operability of theshear enhanced anaerobic digestion apparatus and process to provideelements to allow the successful restart of the digester operation afteran extended shutdown. On shutdown, solid material tends to settle in thedigester 30, potentially building into a layer with a depth sufficientto block at least the outlet of the draft tube 28. The solids buildupinhibits restart of the digester operation since, absent remedialefforts, the solids have a tendency to remain stationary, therebyblocking the pump suction and the draft tube 28, as well as potentiallyblocking the inlet feed. It has been found that by providing liquidcirculation in the bottom of the digester, it will disturb any layer ofbuilt-up solids sufficiently to improve liquid circulation within thedigester 30 and assist start-up. As shown in FIGS. 1 and 5, preferably,the liquid circulation would be supplied by pump 6 and a conduit 36 influid communication with the discharge of the pump 6 and a nozzle 38 onthe digester 30, and would be directed below the deflector plate(s) 25or otherwise in the area above and in proximity to the digester bottom52. This liquid circulation may have any shape that sufficientlydisturbs the layer of built up solids.

[0066] In the event that the liquid circulation is not sufficient toclear the draft tube 28 of solids build up or in lieu of the liquidcirculation, an additional biogas recycle system 21, may be utilized.The biogas recycle system 21, preferably includes internal gas nozzles23, in communication with a conduit 19, to force a gas upwardly aroundand into the draft tube 28 so as to dislodge the solids, beforebeginning normal operation of the nozzle 18. The system may also befurther facilitated by an optional pump 49 to accelerate the flow of gasto nozzles 23.

[0067] It has also been found desirable to use a defoaming spray systemfor the optimum operation of the anaerobic digester in the presentinvention. Such a system prevents the buildup of foam inside thedigester 30 by spraying liquid preferably continuously onto the uppersurface of the mixed liquor in the digester. Such spray preferablycovers the majority or substantially all of the liquid surface. Theimpact of the sprayed liquid on the surface of the liquid level servesto collapse the foam and inhibit foam buildup. Preferably, the liquidsprayed would be mixed liquor from the digester 30. As shown in FIG. 1,at least one spray head 31 is used, and preferably many such sprayheads. This defoaming spray system can be supplemented if necessary withconventional chemical defoamer injection, activated through anappropriate foam sensor, and/or with conventional mechanical methods forfoam destruction.

[0068] The foregoing detailed description refers to the preferredembodiments of the present invention. However, the apparatus accordingto the invention is operable when generally comprising a digester; anysuitable mixing device capable of inducing a circulation andcontributing to an improved mass transfer rate, including but notlimited to the examples discussed above; and any suitable shearingdevice capable of fracturing the solids and introducing them into thedigester, including but not limited to the examples discussed above.

[0069] The area delineated by the dashed rectangle in FIG. 5 may bereplaced by the apparatus shown FIG. 6. FIG. 6 is directed to analternative embodiment of the invention comprising a digester,preferably an anaerobic and shear enhanced digester, coupled with aconcentrator. The apparatus has improved efficiency due to concurrentoperation of the digester and a concentrator. The apparatus includes adigester 30′ preferably including a shear source 71′, such as a shearingnozzle, a Venturi nozzle, an impeller, or some other device capable ofimparting shear to the mixed liquor within the digester, and aconcentrator 62′ in fluid communication with a mixed liquor inlet 77′ ofthe digester and at least one mixed liquor outlet 50′ of the digester30′. The digester 30′ may be the digester 30 above or any digesterpreferably with a source for imparting shear to the mixed liquor in thedigester. If used, the shear source 71′ may be either within or outsideof the digester 30′ or in fluid communication with the digester 30′ solong as it is configured to impart shear to the mixed liquor within thedigester. The concentrator 62′ may be any concentrator that reduces theamount of water in the waste stream, including any of the concentrators62 described above with respect to FIG. 5. The concentrator 62′ and thedigester 30′ can be configured as described above with respect to FIG. 5to effect concurrent concentration and digestion of the mixed liquor.The apparatus may further comprise any of the devices and adopt anyconfiguration described above and will optimize the efficiency,particularly of the anaerobic digestion apparatus. Similar to theconcentrator 62 described above, the concentrator 62′ may comprise apump 70′ and a separator 58′ having an outlet 53′ which is in fluidcommunication with an inlet 77′ of the digester and an inlet 68′ incommunication with a mixed liquor outlet 50′ of the digester. Theconcentrator may also include a conduit 55′ with a first end 57′connected to the outlet 53′ of the separator 58′, and a second end 59′connected to the inlet 77′ of the digester 30′. This conduit preferablyconveys the concentrate from a concentrate side 69′ of the separator 58′back to the digester 30′.

[0070] A waste stream may be digested in various embodiments of ananaerobic digestion apparatus, as described herein, by feeding the wastestream into the digester, preferably taking into consideration theparameters discussed above. The biodegradable material in the wastestream may be reacted with anaerobic bacteria to produce a mixed liquorand a biogas. This reaction may be further optimized by any of themethods described above, if desired, including controlling temperatureand pH, concentrating the mixed liquor, minimizing the entrainment ofgas bubbles, etc. The mixed liquor may be introduced to the digester andany shearing device or method may be used, preferably in communicationwith an inlet of the digester, and the mixed liquor within the digestermay be mixed by any mixing device or method, including those describedherein.

[0071] The invention will now be described in more detail with respectto the following specific, non-limiting examples.

EXAMPLE I

[0072] The data in Table 1 illustrate the improved mass transfer ratesof the digester apparatus of the present invention. The pilot apparatusfor this study was generally the embodiment of the digester apparatus asdepicted in FIG. 1 of the attached drawings, hereinafter referred to asa shear-enhanced anaerobic digester (SEAD digester). The apparatusincluded one two-phase nozzle, a draft tube, and a gas impingementplate, but not the concentrator. The data additionally indicate theimpact of typical polymers on the anaerobic digestion process. The datais taken from a pilot study at a POTW wherein the feed to the SEADdigester was a mixture of primary and secondary sludge that had beenthickened using a conventional belt thickener with the addition ofpolymers. For this study the digester was operated in a once-throughmode without a concentrator. During the study the POTW elected to changethe type of polymer being used as indicated in the table. At the pointindicated in Table 1, the feed to the digester apparatus was changed toa point upstream of the polymer addition and the polymer was purged fromthe system until it was polymer free. TABLE 1 Date Month 3 Month 4 Month5 Month 6 Parameter Chem. A Chem. B Purging Purged Feed Conc. (% TS) 5.14.1 2.3 2.2 HRT (days) 18.1 36.4 11.4 9.5 SRT (days) 18.1 36.4 11.4 9.5Bio-activity (kg/kg/d) 1.3 0.7 1.3 2.0 Rx VFA (meq/L) 5.0 6.9 2.3 0.6

[0073] The above data are monthly average operating data. Chemical A isa cationic water soluble polymer in emulsion. Chemical B is a solutionmannich polymer. The feed concentration is expressed in percent totalsolids (% TS), i.e. lbs. of solids per 100 lbs. of liquid sludge. TheHRT is hydraulic retention time in days, which in a once-through systemequals the solids retention time (SRT). The bioactivity is a measure ofthe volatile solids conversion capacity of the anaerobic biomass and isexpressed as kilograms of volatile solids digested per kilogram ofdigesting bacteria (volatile solids) per day.

[0074] Rx VFA is the concentration of volatile fatty acids in thedigester expressed in milliequivalents per liter (meq/L). Rx VFA is ameasure of the process stability in the digester. Rx VFA readings ofless than 1.0 indicate that the digesting environment is very stable andcould likely perform at even higher mass transfer rates.

[0075] The first column indicates the data for the last month whereinPolymer A was added. The SRT is roughly equivalent to that of anaggressively designed conventional digester. The VFA indicates less thanoptimal stability. The second column indicates the data for thesubsequent month of operation when the POTW switched to a moreeconomical polymer. Performance of the SEAD digester deteriorated asindicated by the much longer SRT as well as the elevated VFA indicatingthat Polymer B was significantly more inhibitory than Polymer A.

[0076] At this point in the study, the location of the feed was moved toallow the SEAD digester to receive the same sludge mix before thickeningand polymer addition. The next column indicates the data for the monthduring which the SEAD digester was gradually purged of the polymer. TheSEAD digester performance increased significantly achieving an SRT belowthe conventional 20-40 days and with a more stable VFA. The final monthof the study reflects the performance of the SEAD digester on the samesludge after the polymer was completely purged from the SEAD system. AnSRT of less than 10 days was achieved with a very stable VFA indication.The study was ended before the most optimal SRT achievable for thisapplication was determined, but the very low VFA indicates that furtherreductions in SRT would likely have been possible.

[0077] The effectiveness of the shear enhanced anaerobic digesterapparatus according to the invention is illustrated by this data. Theimproved mass transfer rates allow for sludge digestion to be performedat an SRT that is 50% of the lower design guideline recommended forconventional systems. The study also identifies that the benefits of theSEAD apparatus are best obtained if the feed stock does not containcommonly used polymers in the thickening process.

[0078] The data in Table 2 are taken from a pilot study on an industrialwaste activated sludge wherein the sludge had been thickened using aconventional thickener with the addition of a cationic polyacrylamide ina water-in-oil emulsion as a polymer. The pilot apparatus for this studywas essentially the embodiment depicted in FIG. 1. TABLE 2 ParameterIndustrial High Rate Egg Shaped Feed Conc. (% TS) 4.1 4.6 5.0 HRT (days)12.2 26 20 SRT (days) 12.2 26 20 Bio-activity (kg/kg/d) 1.2 0.6 0.8 RxVFA (meq/L) 4.5 3.7 N/A.

[0079] Data for Conventional Digesters designated as “High Rate” inTable 2 was taken from WEF (1992): “Manual of Practice No. 8: Design ofMunicipal Wastewater Treatment Plants Volume II: Chapters 13-20”, pp.1261-1263. ISBN 0-943244-85-4. Data for “Egg-Shaped” Digesters in Table2 was taken from Brinkman, Doug and Voss, Denton (1999): “Egg ShapedDigesters, are they all they're cracked up to be?” Water Environment &Technology, November Issue, pp. 28-33. Each of these sources of data ishereby incorporated by reference. The SRT of 12.2 days for the SEADindicates a significant improvement over the design basis SRT forconventional digesters. The elevated VFA indicates that the stability ofthe system was not optimal. Although no study of the effect of thepolymer was undertaken here, without wishing to be bound by any theoryapplicants herein attribute the elevated VFA at least in part to theinhibitory impact of the polymer on the digestion efficiency

[0080] The data in Table 3 are taken from a pilot study on a mixture ofprimary and secondary sludge taken from a POTW system at a point beforethickening and polymer addition. The pilot apparatus for this study wasessentially as depicted in FIG. 5, with the exception that only onetwo-phase nozzle and one draft tube were used, hereinafter designated asa membrane coupled shear enhanced digestion apparatus (MCSEAD digester).TABLE 3 SEAD Transition Extrapolated Parameter Only MCSEAD MCSEAD FeedConc. (% TS) 0.5 0.5 0.5 HRT (days) 11 5 1 SRT (days) 11 11 11Bio-activity (kg/kg/d) 1.4 2.0 1.4 Rx VFA (meq/L) 0.6 2.0 <1.0

[0081] The first column indicates the performance obtained on theunthickened feed without the concentrator in operation. The SRT againindicates that the improved mass transfer rates of the SEAD apparatusaccording to the invention are beneficial to the digestion process. Thelow VFA indicates that further reductions in SRT might be possible. Theconcentrator was then activated and the same sludge feed wasconcurrently concentrated and digested. Concentration was achieved witha membrane separation system without the addition of polymers asindicated in the preferred embodiment. The second column indicates theperformance with the concentrator operating during a transition periodas the mixed liquor TSS is increasing. As the quantity of digestingbacteria accumulated in the digester, increased demand on the existingbacteria is indicated by the increase in Bioactivity and slightlyelevated VFA. When the concentration transition is complete, equilibriumwill be restored and the extrapolated results are indicated in columnthree. Data for the equilibrium condition were not obtained due tomechanical limitations at the pilot scale. The membrane separatorperformed effectively as evidenced by the reduction of HRT at a constantfeed concentration and SRT.

[0082] Sludge digester systems are typically once through systemsdesigned based on SRT and HRT. The solids concentration of raw sludge istypically about 1% TS. In order to keep digester volume reasonable,sludge is often thickened to about 5% TS before digestion. Table 2 showsthat the embodiment of the digester of the apparatus of the presentinvention including a shearing device and a mixing device can achievethe same process performance with prethickened sludge as a conventionalhigh rate or egg-shaped digester, but at a considerably reduced SRT andHRT.

[0083] Further, Table 1 shows that thickening polymers can be moderatelyto severely inhibitory to the anaerobic bacteria in the applicants'digester having a shearing device and a mixing device, and thatelimination of such polymers improves process stability and bacterialactivity. Mere removal of the polymer is not a practical way in which toimprove such process stability and bacterial activity because it wouldrequire treatment of dilute sludge in such a digester apparatus, whicheven at reduced SRT and HRT would require a large digester volume.Further, prethickening raw sludge from 1% TS to 5% TS with a membraneseparator upstream of the digester is difficult, as discussed above,because the presence of hydrophilic compounds in sludge can preventefficient performance of the upstream membrane. Accordingly, applicants'embodiment further including concurrent digestion and concentration ofthe mixed liquor permits control of SRT independently of HRT, making itpossible to digest dilute sludge without use of inhibitory polymers andusing a digester volume similar to or less than the digester volumesassociated with the prethickened sludge as used in Table 2. Table 3illustrates that for a given digester volume, applicants digesterembodiment using the addition of a membrane concentrator according tothe invention permits maintaining an SRT of 11 days while increasing thedigester throughput by a factor of 11, as shown by the decrease in HRTfrom 11 days to 1 day.

[0084] It will be appreciated by those skilled in the art that changescould be made to the embodiments described above without departing fromthe broad inventive concept thereof. It is understood, therefore, thatthis invention in not limited to the particular embodiments disclosed,but is intended to cover modifications within the spirit and scope ofthe present invention defined by the appended claims.

We claim:
 1. An anaerobic solids digestion apparatus comprising: (a) adigester; (b) a mixing device in the digester capable of directing aflow of a mixed liquor within the digester; and (c) a shearing device incommunication with a mixed liquor inlet to the digester, the shearingdevice being capable of imparting shear to a mixed liquor within thedigester.
 2. The anaerobic solids digestion apparatus according to claim1, wherein the mixing device comprises a draft tube positioned in thedigester and having an upper inlet and a lower outlet.
 3. The anaerobicsolids digestion apparatus according to claim 2, wherein the draft tubehas a length measured along a longitudinal axis of the draft tube ofabout 50% to about 90% of the digester liquid depth measured along alongitudinal axis of the digester.
 4. The anaerobic solids digestionapparatus according to claim 2, further comprising an deflector platebelow the lower outlet of the draft tube and positioned to at leastpartially block gas from the lower outlet of the draft tube fromentering a liquor outlet of the digester.
 5. The anaerobic solidsdigestion apparatus according to claim 1, wherein the shearing devicecomprises a shearing nozzle.
 6. The anaerobic solids digestion apparatusaccording to claim 5, wherein the nozzle comprises a gas inlet, a liquidinlet, an outlet, an interior surface, and a gas tube having an exteriorsurface, the gas tube extending from the nozzle gas inlet to the nozzleoutlet, wherein a generally annular space is defined by the exteriorsurface of the gas tube and the interior surface of the nozzle.
 7. Theanaerobic solids digestion apparatus according to claim 6, furthercomprising a biogas source in communication with the gas inlet of thenozzle.
 8. The anaerobic solids digestion apparatus according to claim7, wherein the biogas source is a biogas recycle system capable ofremoving biogas from a biogas collection area in the digester anddirecting it to the gas inlet of the nozzle.
 9. The anaerobic solidsdigestion apparatus according to claim 8, wherein the biogas recyclesystem comprises a conduit having a first end forming an outlet forbiogas removed from the gas collection area and a second end incommunication with the gas inlet of the nozzle.
 10. The anaerobic solidsdigestion apparatus according to claim 7, wherein the biogas sourceincludes methane and carbon dioxide.
 11. The anaerobic solids digestionapparatus according to claim 10, wherein the biogas source includesnitrogen.
 12. The anaerobic solids digestion apparatus according toclaim 10, wherein the biogas source includes a small amount of oxygensufficient to modulate the oxidation-reduction potential (ORP) of themixed liquor.
 13. The anaerobic solids digestion apparatus according toclaim 5, further comprising a recirculation system, wherein the nozzlefurther comprises a liquid inlet and the recirculation system is influid communication with a mixed liquor recirculation outlet of thedigester capable of recirculating the mixed liquor from the mixed liquorrecirculation outlet of the digester to the liquid inlet of the nozzle.14. The anaerobic solids digestion apparatus according to claim 13,wherein the recirculation system comprises a pump and a first conduithaving a first end connected to the pump and a second end in fluidcommunication with the liquid inlet of the nozzle and a second conduitwith a first end connected to the mixed liquor recirculation outlet anda second end connected to an inlet of the pump.
 15. The anaerobic solidsdigestion apparatus according to claim 13, wherein the recirculationsystem is capable of providing a liquid circulation within the digester.16. The anaerobic solids digestion apparatus according to claim 13,wherein the recirculation system further comprises a temperature controlmechanism.
 17. The anaerobic solids digestion apparatus according toclaim 16, wherein the temperature control mechanism is a heat exchanger.18. The anaerobic solids digestion apparatus according to claim 16,wherein the temperature control mechanism is a steam injector.
 19. Theanaerobic solids digestion apparatus according to claim 1, furthercomprising a concentrator in fluid communication with the mixed liquorinlet of the digester and at least one mixed liquor outlet of thedigester.
 20. The anaerobic solids digestion apparatus according toclaim 19, wherein the concentrator comprises a pump and a separatorhaving an outlet in fluid communication with a the mixed liquor inlet ofthe digester.
 21. The anaerobic solids digestion apparatus according toclaim 20, wherein the concentrator further comprises a conduit with afirst end in fluid communication with the mixed liquor inlet of thedigester and a second end in fluid communication with an outlet of theseparator.
 22. The anaerobic solids digestion apparatus according toclaim 20, wherein the separator is a membrane separator.
 23. Theanaerobic solids digestion apparatus according to claim 1, wherein thedigester comprises a gas de-entrainment zone in communication with amixed liquor outlet of the digester.
 24. The anaerobic solids digestionapparatus according to claim 1, wherein the digester has a solidsretention time of about 2 to about 20 days.
 25. The anaerobic solidsdigestion apparatus according to claim 24, wherein the digester has asolids retention time of about 6 to about 12 days.
 26. The anaerobicsolids digestion apparatus according to claim 1, further comprising a pHadjustment system (capable of maintaining the mixed liquor in thedigester at a pH of about 6 to about
 8. 27. An anaerobic solidsdigestion apparatus comprising: (a) a digester comprising a shear sourcecapable of imparting shear to a mixed liquor within the digester; and(b) a concentrator in fluid communication with a mixed liquor inlet ofthe digester and at least one mixed liquor outlet of the digester,wherein the concentrator and digester are configured to allow forconcurrent concentration and digestion of a mixed liquor.
 28. Theanaerobic solids digestion apparatus according to claim 27, furthercomprising a draft tube comprising an upper inlet and a lower outlet andpositioned in the digester, wherein the draft tube is capable ofdirecting a flow of a mixed liquor.
 29. The anaerobic solids digestionapparatus according to claim 27, wherein the shear source is a shearingnozzle for introducing mixed liquor into the digester.
 30. The anaerobicsolids digestion apparatus according to claim 29, wherein the nozzlecomprises a gas inlet, a liquid inlet, an outlet, and an interiorsurface, the nozzle further comprising a gas tube having an exteriorsurface, the gas tube extending from the nozzle gas inlet to the nozzleoutlet, wherein a generally annular space is defined by the exteriorsurface of the gas tube and the interior surface of the nozzle.
 31. Theanaerobic solids digestion apparatus according to claim 29, furthercomprising a biogas source, wherein the nozzle further comprises a gasinlet and the biogas source is a biogas recycle system capable ofremoving biogas from a biogas collection area in the digester anddirecting it the gas inlet of the nozzle.
 32. The anaerobic solidsdigestion apparatus according to claim 27, further comprising a biogassource.
 33. The anaerobic solids digestion apparatus according to claim27, wherein the concentrator comprises a pump and a separator having anoutlet in fluid communication with a mixed liquor inlet of the digester.34. The anaerobic solids digestion apparatus according to claim 33,wherein the concentrator further comprises a conduit with a first end influid communication with the mixed liquor inlet of the digester and asecond end in fluid communication with an outlet of the separator. 35.The anaerobic solids digestion apparatus according to claim 33, whereinthe separator is a membrane separator.
 36. An anaerobic solids digestionapparatus comprising: (a) a digester; (b) at least one draft tubepositioned in the digester and capable of directing a flow of a mixedliquor and comprising an upper inlet and a lower outlet; (c) at leastone nozzle comprising a gas inlet, a liquid inlet, an outlet and aninterior surface, the nozzle further comprising a gas tube having anexterior surface, the tube extending from the nozzle gas inlet to thenozzle outlet, wherein a generally annular space is defined between theexterior surface of the gas tube and the interior surface of the nozzle;and (d) a biogas source in communication with the gas inlet of thenozzle.
 37. A method for digesting a waste stream in an anaerobic solidsdigestion apparatus, the method comprising: (a) feeding a waste streamcomprising anaerobically biodegradable solids to a digester; (b)reacting the anaerobically biodegradable solids in the waste stream withanaerobic bacteria in the digester to reduce an amount of thebiodegradable solids, thereby producing a mixed liquor and a biogas; (c)introducing a mixed liquor to the digester through a shearing device;and (d) mixing the mixed liquor within the digester.
 38. The methodaccording to claim,37, wherein the shearing device comprises a nozzle,wherein the mixed liquor flows through the nozzle in a generally annularspace defined by an exterior surface of a gas flow tube positionedwithin the nozzle and an interior surface of the nozzle.
 39. The methodaccording to claim 37, further comprising removing a portion of thebiogas from the digester and introducing the portion of the biogas intoa nozzle gas inlet.
 40. The method according to claim 37, furthercomprising introducing the portion of the biogas and mixed liquor fromthe nozzle into a draft tube within the digester and mixing the portionof the biogas and mixed liquor, thereby inducing internal circulationwithin the digester as the biogas and mixed liquor flow downwardlythrough the draft tube.
 41. The method according to claim 37, furthercomprising concentrating the mixed liquor.
 42. The method according toclaim 41, wherein the steps of reacting the anaerobically biodegradablesolids and concentrating the mixed liquor occur concurrently.
 43. Themethod according to claim 37, wherein the method minimizes the need foruse of a polymer that inhibits biological activity in a waste stream.44. The method according to claim 37, further comprising maintaining amixed liquor pH of about 6 to about
 8. 45. The method according to claim37, further comprising maintaining a mixed liquor temperature of about80° F. (25° C.) to about 105° F. (40° C.).
 46. The method according toclaim 37, further comprising maintaining a mixed liquor temperature ofabout 125° F. (50° C.) to about 145° F. (60° C.).
 47. The methodaccording to claim 37, further comprising minimizing the entrainment ofgas bubbles in the recirculation system.
 48. The method according toclaim 37, further comprising venting biogas from the gas collection areaof the digester.
 49. The method according to claim 37, furthercomprising minimizing foam in the digester.
 50. The method according toclaim 37, further comprising removing scum from the digester.
 51. Amethod for improving the efficiency of an anaerobic solids digestionapparatus comprising: a) feeding a waste stream comprising anaerobicallybiodegradable solids to a digester; b) reacting the anaerobicallybiodegradable solids in the waste stream with anaerobic bacteria in thedigester to reduce an amount of the biodegradable solids, therebyproducing a mixed liquor and a biogas; c) imparting a shearing force tothe mixed liquor in the digester; and d) concentrating the mixed liquor,wherein the steps of reacting the anaerobically biodegradable solids andconcentrating the mixed liquor occur concurrently.
 52. A method forminimizing the need for use of a polymer that inhibits biologicalactivity in a waste stream in a digestion apparatus, the methodcomprising: (a) feeding a waste stream to a digester, wherein a portionof the waste stream is biodegradable; (b) reacting the biodegradableportion in the waste stream with bacteria in the digester to produce amixed liquor and gas; and (c) concentrating the mixed liquor with amembrane separator, wherein the steps of reacting the biodegradableportion in the waste stream and concentrating the mixed liquor occurconcurrently.
 53. The method according to claim 52, wherein the wastestream comprises anaerobically biodegradable solids, and the methodfurther requires reacting the biodegradable solids in the waste streamwith anaerobic bacteria in the digester to produce a mixed liquor and abiogas, and wherein the steps of reacting the anaerobicallybiodegradable solids an concentrating the mixed liquor occurconcurrently.
 54. The method according to claim 52, wherein the need foruse of a polymer that inhibits biological activity in a waste stream inan anaerobic solids digestion apparatus is eliminated.